1. Field of the Invention
The present invention relates to a method of and an apparatus for detecting a crack in a pair of piezoelectric elements used for a piezoelectric actuator or the like.
2. Description of the Related Art
There is a dual-actuator system as a technique to realize high density of a hard disk drive (HDD) as disclosed in JP2002-50140A. The dual-actuator system has a head suspension with a piezoelectric actuator in addition to a voice coil motor.
The voice coil motor drives a carriage to which the head suspension is attached to turn the head suspension. The piezoelectric actuator includes piezoelectric elements being arranged at an intermediate portion between a base plate and a load beam or at a head portion and being deformable according to voltage applied thereto to minutely drive the head portion in a sway direction. The dual-actuator system, therefore, precisely positions a magnetic head on the head portion.
The head suspension used in the dual-actuator system needs to be thinned according to downsizing of the HDD. Accordingly, the piezoelectric element used for the piezoelectric actuator also needs to be thinned.
The thinned piezoelectric element is likely to cause micro cracks by external force received, for example, at the time of production of the piezoelectric element or assembly of the same to the head suspension. A cracked piezoelectric element is concerned about reduction of long-term reliability and must be discarded as a defective.
A micro crack, however, is hard to be found from appearance using a stereoscopic microscope in general. The hardness is promoted by electrodes formed by gold plating, platinum plating or the like on the surfaces of the piezoelectric element.
Further, such a micro crack is hard to be found by measuring electric characteristics, either. Namely, a piezoelectric element is subjected to a performance test at which, for example, capacitance is measured after assembled into a head suspension. The micro crack, however, do not cause a change in the capacitance.
On the other hand, there are methods of detecting a crack of a piezoelectric element in which one conducts comparison between patterns in frequency characteristics of impedances and/or phases and the other uses optical transparency of a piezoelectric element as disclosed in, for example, JPH06-003305A and JP2002-367306A.
These methods, however, hardly determine presence or absence of a crack or are hardly applied for practical purposes. As a result, these methods do not surely detect a micro crack.